Certain embodiments of the present invention relate to a medical diagnostic ultrasound system for imaging blood flow within a human subject. More particularly, certain embodiments relate to a method and apparatus for automatically controlling spectral Doppler imaging for a sample gate within a scan plane.
Ultrasound systems to detect and image blood flow based on the Doppler effect are well established. An operator typically places a sampling gate over a location in an image to be measured in a subject. For example, the sampling gate may be placed over an artery in the subject""s kidney. Ultrasound energy is transmitted into the subject by an emitting transducer and reflected energy is received by the transducer in the form of waves. To measure the velocity of blood flow within a sampling gate within the subject, the phase and amplitude of the reflected waves are detected and the information is compared to a reference frequency to discern the Doppler shifts (frequency shifts) experienced by the reflected waves by the moving blood cells within the sampling gate.
For a given instant in time, the Doppler information for a given sampling gate may extend over a range of frequencies. The information is represented by the ultrasound system as a spectral line of frequency or velocity information. The spectral line of information represents the estimated instantaneous velocity of the blood flow within the sampling gate. A spectral display may be formed that plots the spectral line for each instant in time over, for example, a cardiac cycle. The resultant display format is Doppler frequency (or velocity) versus time. The spectral information may be displayed in real time using grey-scale coding to represent the signal strength or power in the spectral signal at the various frequencies.
The data in each spectral line comprises a plurality of frequency bins and the signal strength (power) associated with each frequency bin is displayed in a corresponding pixel location on the display. All of the spectral lines taken together form a spectrogram. Sometimes the spectrogram may be aliased. When a spectrogram is aliased, the displayed spectrogram is wrapped around the velocity scale limit such that positive velocity values may appear negative and vice versa. If the total spectral bandwidth of the signal is less than the pulse repetition frequency (PRF), a simple shift of the baseline may effectively unwrap the spectrum. If the spectral bandwidth is larger than the PRF, the spectrum may not be unwrapped by adjusting the baseline position. Instead, the velocity scale, PRF, should be increased. Also, the vertical orientation of the Doppler spectrum may provide for better intuitive visualization if the spectrum is inverted.
Certain standard diagnostic Doppler indices are based on frequency estimates at a particular segment in the cardiac cycle such as peak systole or end diastole. An operator often desires to trace the peaks of the spectral lines across the display so the indices may be calculated. Manual tracing is often very difficult, time consuming and inaccurate.
Ultrasound systems have been proposed that automatically eliminate aliasing in Doppler spectral images by adjusting the PRF (velocity scale) based on pre-calculated noise levels. The pre-calculated noise levels are used to predict if the spectral waveform is aliased and/or inverted. The algorithm shifts the baseline and/or inverts the velocity scale to position the spectral waveform in the desired portion in the timeline display, or increases the PRF to expand the velocity scale in order to eliminate aliasing in the spectral image. The predicted noise levels are also used to determine the peaks of the spectral lines so a trace may be drawn on the display. The performance of the method relies on the accuracy of the pre-calculated noise level estimates.
For example, one method described in U.S. Pat. No. 5,935,074 predicts the mean noise level in the background of the spectral image. The pre-amp Johnson noise is calculated. The noise is adjusted for all of the filters in the Doppler signal path. The quantization noise due to analog-to-digital conversion is added to the noise estimate. The noise is summed over all active receiving channels taking into account transducer array apodization effects. The noise is converted to the mean noise level in the spectral display through dynamic range compression. The method, however, is system configuration dependent. Also, if the noise prediction is inaccurate at any stage in the signal path, then the entire prediction is suspect.
A need exists for an approach to automatically control certain parameters associated with spectral Doppler imaging based only on noise levels and signal levels actually present in the lines of Doppler spectral data without relying on any pre-calculated predictions of noise at various system stages and without assuming that a certain region of an image contains only noise. A need exists for a way to automatically eliminate aliasing, and adjust the baseline and orientation of the spectral signal, if necessary, to present a visually desirable display of the spectral signal to an operator. A need also exists to generate a spectral trace of the spectral signal based only on the lines of spectral Doppler data and estimated noise levels.
An embodiment of the present invention provides an ultrasound system for imaging velocity information of a location within a subject, designated by a sampling gate, by automatically controlling certain parameters that affect Doppler spectral imaging corresponding to the location. Automatic adjustment of certain parameters associated with Doppler spectral imaging results in eliminating aliasing, setting the baseline to a more desirable location on the display, and inverting the Doppler spectral image. Spectral lines of Doppler data generated by the ultrasound system are acquired. The presence of aliasing and estimates of noise levels and signal boundaries are determined by the system from the spectral lines of Doppler data. The system automatically adjusts certain parameters such as pulse repetition frequency (PRF), baseline shift, and spectrum orientation in response to aliasing, noise levels, and signal boundaries. The system also determines a positive signal boundary and a negative signal boundary for each spectral line of Doppler data and processes the signal boundary data to display a spectral trace corresponding to the edges of the spectral lines.
An apparatus is provided for controlling certain parameters associated with a Doppler spectral display generated by an ultrasound system corresponding to a location within a scan plane designated by a sample gate. The apparatus includes a transducer for transmitting and receiving ultrasound signals and a beamformer for deriving data samples representative of the reflected ultrasound signals from a designated location within a scan plane. Also, a Doppler processing module for generating raw spectral Doppler data from the data samples is provided. A scan conversion module scan converts the raw spectral Doppler data and a data processing module analyzes a plurality of spectral lines of Doppler data and automatically adjusts certain system parameters to control visualization of a Doppler spectral image. A display architecture displays the Doppler spectral image corresponding to the designated location within the scan plane. The data processing module may also generate signal boundary data corresponding to the edges of the spectral lines of Doppler data and the display architecture may generate and display a spectral trace corresponding to the edges of the spectral lines.
A method is also provided for controlling certain parameters associated with a Doppler spectral display generated by an ultrasound system corresponding to a location within a scan plane designated by a sample gate. The method includes acquiring a plurality of spectral lines of Doppler data generated by the ultrasound system. The presence of aliasing is determined from the plurality of spectral lines of Doppler data. Also, noise levels and signal boundaries are estimated from the plurality of spectral lines of Doppler data. System parameters are automatically adjusted, if necessary, including pulse repetition frequency, baseline shift, and spectrum orientation in response to a determination of the presence of aliasing and an estimation of noise levels and signal boundaries. A spectral trace corresponding to the edges of the spectral lines may also be generated and displayed.
Certain embodiments of the present invention afford an approach to automatically control certain parameters associated with the generation of a Doppler spectral image. Automatic control allows the system to eliminate aliasing, set the baseline, and invert the image, if necessary, by processing only the spectral lines of Doppler data corresponding to the designated location. The generation of a spectral trace corresponding to the edges of the spectral lines is also achieved.